H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

We are currently installing the new upgraded fuel system on the SportCruiser.Basically it involves a check valve, reduced hosing, new hose routing, and the new banjo fittings.

The Case for No Fuel Return Restrictor:As per parts list from US Sport Aircraft, IPC, and photos from those that have upgraded their fuel system, there is no fuel restrictor on the top banjo fitting to the fuel return line. I have verified with very credible owner / mechanics of Evektor aircraft that they do not have a restrictor on their fuel return line either.

The Case For a Fuel Return Restrictor:Others are adamant that a fuel return line restrictor is required saying, "If there is no restrictor you will have no fuel pressure." The SportCruiser maintence manual schematic notes that there is a restrictor on the return line (however that schematic upon further inspection is very different from real-world installations). A case has been made that Rotax requires a restrictor.

It's the weekend and I haven't been able to receive a definitive answer from US Sport Aircraft, and suspect the possibility of inconsistencies.

Personally, I believe that there is no fuel line return restrictor on the new fuel system.Yet I also have respect for those making the case for the fuel line restrictor.

Posting here, hoping that someone with further experience on the issue can comment and/or help resolve.

I'm familiar with fuel return lines. Fuel injected engines often utilize fuel pumps that pump at a higher rate than needed, and excess is returned to one tank or the other*.

What is the purpose/function of a return line on a carbureted engine? It's not something I've heard of before. My Sky Arrow does not seem to show any such lines, and it is similar in that the engine is above the fuel tank.

*I had to deal with this when dealing with ferry tanks. I had to burn down a wing tank substantially before switching to the ferry tank. Otherwise the excess fuel would cause the plane's tank to overflow from the excess fuel being dumped into a full tank.

FastEddieB wrote:What is the purpose/function of a return line on a carbureted engine?.

On those installations that include an electric boost pump, when both it and the mechanical fuel pump are operating simultaneously (e.g. for takeoff), the two can provide more fuel than needed to keep the float bowls full. In those cases, the excess fuel is returned to a tank.

FastEddieB wrote:What is the purpose/function of a return line on a carbureted engine?.

On those installations that include an electric boost pump, when both it and the mechanical fuel pump are operating simultaneously (e.g. for takeoff), the two can provide more fuel than needed to keep the float bowls full. In those cases, the excess fuel is returned to a tank.

Got it.

My Sky Arrow fits into that category, but pretty sure it has no such line.

Back in the old days, automobiles used an engine-mounted pump to "suck" fuel out of the gas tank. This resulted in a pressure lower than ambient in the fuel line between the gas tank and the pump inlet. Since part of this line was in the engine compartment where things got hot, fuel in the hot, low-pressure fuel line tended to boil. The pump and carburetor were designed to operate on liquid not gas, so when the gasoline boiled in the fuel line you got what they called "vapor lock". The engine would starve for fuel when this happened. Some of you older guys may remember this.

If you think I'm making this up, take a glass syringe, fill it half full of gasoline, block the inlet, pull the plunger back and see what happens.

One way to deal with vapor lock was to install a return line to the fuel tank. This did two things. 1. It supposedly allowed vapor (if any) to return to the fuel tank. Vapor is less viscous than liquid so vapor went through the restriction in the return line preferentially. 2. It kept cool fuel circulating through the system, reducing the tendency for the fuel to boil. The ROTAX engines use this old-fashioned setup... fuel pump mounted on the engine, return line to avoid vapor lock. There IS a restrictor in the return line out of the fuel manifold that sits on top of the engine. If there were no restrictor, fuel would just go straight back to the tank and you would have little or no pressure at the carburetor inlets. The restrictor is specified to allow enough fuel (and vapor) to return to the tank while maintaining the desired pressure at the carb inlets.

Modern cars generally have an electric pump mounted in the fuel tank. This way the entire fuel system from the tank to the carburetor (or fuel injectors) is pressurized. The higher pressure keeps the fuel from boiling and so no vapor lock. This is a much better way of doing things and it should be standard on our airplanes.

You ask "Why do certified airplanes with Lycomings and Continentals not suffer from vapor locking/low fuel pressure?" I don't know the answer to that. The ones I've seen have engine-mounted fuel pumps and no return line. Maybe it IS the return line that's causing our low fuel pressure indications.

FastEddieB wrote:What is the purpose/function of a return line on a carbureted engine?.

On those installations that include an electric boost pump, when both it and the mechanical fuel pump are operating simultaneously (e.g. for takeoff), the two can provide more fuel than needed to keep the float bowls full. In those cases, the excess fuel is returned to a tank.

The CT doesn't have a boost pump, but has the return line. I have always been told it is to help eliminate vapor lock.

It might help with excessive fuel, but that's not its original purpose. Remember the orifice is only a Mikuni #35 idle jet. That hole is so tiny it takes one strand of a #14-#16 gauge wire to go down through the center. I passes a very small amount of fuel. The pump(s) can flow a whole lot more volume of fuel than that tiny orifice can control. The pressure in the fuel lines is the same with or without the recirculation line. The orifice has to be small enough to maintain fuel pressure and small enough not to adversely affect flow. If the recirculation line was an absolute to control fuel flow then all the Rotax 912's in the world without a recirculation line would all be flooding which isn't the case. So flow in this case isn't the main issue.The carbs should be correctly configured to hold back any pressure that is delivered from the pump(s). If not they would overflow and flood with or without a recirculation line.It's main purpose is fuel vaporization within the fuel lines. Depending on rpm and whether an owner has his fuel lines in fire sleeve (like they should be) the fuel flow can be slow enough to allow for vapor lock due to fuel line heat absorption and retention and possibly being exacerbated by poor air flow within the cowl. Allowing for extra fuel to flow during its operating times allows cooler fuel to stay within the lines which can help prevent vapor lock which in turn may prevent an engine out or a stumble at the worst possible time. This can also help with cooler fuel in the carbs so vapors don't try and form there either. Approaches to landing with reduced fuel flow is one of those times you don't want vapor lock. In the earlier days with more open air engines this wasn't an issue. In today's aircraft with most engines in cowls and snug fits this can cause heat retention. Some aircraft have good airflow and some don't. Some owners and aircraft MFG's put fuel hose in fire sleeve which is an ASTM standard, but some don't. Keeping heat out of the fuel line isn't the fire sleeves purpose, but it helps compared to a bare line.

bstrachan wrote: You ask "Why do certified airplanes with Lycomings and Continentals not suffer from vapor locking/low fuel pressure?" I don't know the answer to that. The ones I've seen have engine-mounted fuel pumps and no return line. Maybe it IS the return line that's causing our low fuel pressure indications.

That is the Mikuni idle jet inside the Banjo bolt. It has been cut off to fit the inside length. It's the hole down through the center of the idle jet that is important and would take a very small piece of debris to clog. This is a fairly typical place for them. Some people solder them in place.

Roger, thanks for the very concise explanation of vapor lock issues. I don't doubt your information about the use of an idle jet as a restrictors orfice, but am unable to locate one in either of the Evektors I maintain. The SportStar doesn't use the standard Rotax fuel distribution block, but rather straight-thru aluminum t-connectors. Any ideas where the orfice might be hidden, if in fact it's used?

Hi Paul,There has to be some sort of a restrictor in a recirculation hose or there would be no fuel pressure. I'm not familiar enough with either of those aircraft so I don't know its location. If it isn't at the beginning of the recirculation line then look at the other end.

The information for the installation is in the Rotax Installation manual, section 73-00-00 page 4